This invention relates to an hydraulically operated impact device, e.g. rock drill, comprising a reciprocably driven hammer piston arranged to impact upon an anvil means of a tool member, a supporting member for axially supporting the tool member, and a support piston that is slidable in a cylinder and subject to the hydraulic pressure in a pressure chamber in order to bias said supporting member into a defined forward end position. The pressure chamber is connected to a source of high pressure fluid and narrow clearances between the relatively moving surfaces of the support piston and its cylinder form narrow leak passages from said pressure chamber. The support piston and the pressure chamber form a damping device that reduces the stress on the housing of the impact device by dampening the reflected shock waves that propagate from the bit of the tool rearwardly through the tool which can be the drill stem of the rock drill or the chisel of a jack hammer or the like.
An impact device of this kind is described in U.S. Pat. No. 4,073,350. Because of the tolerances, it is unavoidable that the narrow clearances vary a great deal between rock drills of the same production line. Since the leakage varies with the cube of the width of the clearances, the leakage will vary a great deal. The leakage is a loss of energy which reduces the overall efficiency of the impact device.
One object of the invention is to control the leak flow out of the dampening device and simultaneously to give the damping device long service intervals. This will be achieved by the features defined in the characterizing parts of the claims.